1. Field of the Invention
This invention relates to electrical cable for use with submersible pumps used in oil wells, and in particular to electrical cables that are resistant to hydrogen sulfide gas.
2. Description of the Prior Art
This invention concerns electrical submersible pump cables, commonly referred to as ESP cables, used to power downhole electrical motors for submersible pumps in oil wells. Submersible pumps provide an economical method of pumping large volumes of production fluids from wells that are often several thousand feet deep and often under high temperatures and pressures. The production fluids found in these wells will often contain large amounts of dissolved gases such as methane, carbon dioxide and hydrogen sulfide. Electrical cable used to power these pumps must be specifically designed to withstand exposure to these gases and to the damaging effects of decompression which occurs when the pressure within the well is rapidly reduced such as when the submersible pump and electrical cable are pulled to the surface for servicing.
A typical electrical cable for a submersible pump consists of a copper conductor which is surrounded by an insulating material. Several conductors may be used for each cable, each conductor having a layer of insulating material surrounding it. Typically three insulated conductors are used for each cable. The insulated conductors are sheathed in an extruded elastomeric jacket to resist penetration by well fluids with a layer of metal outer armor surrounding the elastomeric jacket.
For most wells the use of electrical cable constructed as above is adequate. However, wells containing hydrogen sulfide gas in sufficient quantities require special considerations. Hydrogen sulfide has been known to permeate the insulation and chemically react with the copper used in the electrical cable to form copper sulfide. Copper sulfide is a non-conductive material that has a lower density than that of metallic copper. When hydrogen sulfide gas permeates the insulation surrounding the copper conductor, the hydrogen sulfide reacts with the copper to form the copper sulfide and causes the conductor to swell. The ability of the conductor to conduct electricity is also reduced because of the copper sulfide.
The conventional method of preventing hydrogen sulfide from permeating the insulation and reacting with the copper conductor is to extrude an impermeable, continuous layer of lead around the insulation to prevent the hydrogen sulfide from penetrating the insulating material. The conventional electrical cable with the lead sheath has many serious drawbacks and limitations however. The lead sheath makes the electrical cable heavy and difficult to handle. It is easily damaged during flexing, with the lead becoming increasingly brittle in colder environments. The lead has a poor fatigue resistance which limits the number of times the electrical cable can be reinstalled in a well. Because the lead sheath must be quite thick, typically 0.040 inches, there is less room for insulation in the cable construction. This often requires the voltage rating to be lowered or causes the insulation to be over-stressed. Repairing and splicing of the lead sheath is difficult and requires a new lead sheath to be placed over the splice and hermetically sealed around each conductor. This is often difficult to achieve with soldering techniques.
One of the major problems associated with conventional electrical cables employing continuous lead sheaths is decompression. The extrusion, handling and splicing eventually creates small holes or cracks along the lead layer so that gases penetrate into the insulation when the cable is introduced into a gas well. Gases will continue to permeate the insulation until the pressure of the dissolved gases in the intermolecular spaces of the insulation and the pressure of the gases in the well fluid reach equilibrium. Decompression occurs when the pressure outside the cable is reduced causing the dissolved gases inside the insulation to expand and escape from the cable until a new, lower pressure equilibrium condition is established.
The two principle sources of decompression occur when a reduction in fluid column height within the well is achieved due to pump activation or when the cable is removed from the well. The rate of pressure change in either case depends on many variables such as reservoir characteristics and pull rates. The rapid reduction in pressure can easily damage the electrical cable insulation. When the pressure is reduced, the dissolved gases tend to expand, just as when the pressure is relieved when opening a soda bottle. If the pressure change is rapid enough, bubbles will form inside the insulation causing microscopic tears in the insulation. In some cases, decompression can be so severe as to cause holes to "blow out" in the insulation rendering the cable useless.
During decompression of conventional lead sheathed cables, gases escaping from the insulation tend to build up beneath the lead layer. Because the gas cannot escape fast enough through the small holes through which the gas entered, the lead sheath often begins to stretch and rupture. This leads to a sudden decompression of the insulation, causing blow outs and other physical and chemical damage.
What is needed is an electrical cable for a submersible well pump which is resistant to hydrogen sulfide gas but that is lighter than conventional lead covered cables, is less likely to fatigue, is easily repaired or spliced, and is less inclined to rupture from internal gas pressure during decompression.